1. Field of the Invention
The present invention relates in general to erasable optical memories. It relates more particularly to the inscription of data into, and the reading or erasing of such data from, a layer of such a memory.
2. Description of the Related Art
One widely used form of an optical memory is the optical disk. Optical disks are made from a disk of a rigid and generally transparent material which sandwiches a thin film of a light-reactive material. The disk rotates at a relatively high speed and passes in front of a reading head which detects the data which has been recorded in the surface layer. For some optical disks, the data is stored in the form of perforations in the surface layer. An optical reader reads the data by passing a laster beam through the transparent support and through the data storage holes of the optical disk or by reflecting a laser beam through the transparent support of the disk onto the surface layer. The data can also be stored in the form of surface deformations in which reflections from the deformations cause different readings.
For erasable optical memories such as optical disks, two types of structures exist for the inscription layer: non-erasable structures such as perforated metal layers, and erasable structures such as photographic structures, photoresists, and ablative thin films. Erasable structures use amorphous, metallic, or semiconducting materials, or magnetooptic, photochromic, photoferroelectric, thermoplastic, or photodichroic materials, or chalcogenide films. These materials change their optical properties after laser beam inscription. The general disadvantage of such erasable structures in the past has been a low signal-to-noise ratio, and the inability to erase individual bits of digital data. It is believed that until the present invention, an entire optical disk, an entire section of the disk, or a track of the disk had to be erased before rewriting could occur.
In one prior memory, a laser beam applies a heat pulse during a write operation to cause a highly expansible central layer to expand, raise, and disengage from a lower supporting surface, and to deform a layer of a relatively inexpansible marmem alloy superimposed on the central layer. The central layer then cools, lowers, and disengages from the marmem alloy protuberance. To erase the protuberance, a more powerful laser beam heat pulse changes the marmem alloy from its martensitic phase to a crystallographic phase, thus returning the marmem alloy to its original flat shape. The flat marmem alloy once again engages the highly expansible lower layer.
The continuing engagement-disengagement cycle between the upper and lower layers during write-erase operations eventually results in partial but permanent disengagement of the layers. Applicant believes the permanent disengagement prevents a deformation in the central layer from producing a protuberance of sufficient size in the upper marmen layer to give an acceptable signal-to-noise ratio when the optical reader reads the stored data.
The energy required to operate the laser for the writing and erasing operations is a function of the temperature and time length of the laser pulses. Typical writing temperatures have been around 1000.degree. F., and erasing temperatures still higher. Thus, the high energy required to operate the later has greatly limited the use of this technology.
Thus, a need remains for erasable optical memories that retain an acceptable signal-to-noise ratio after repeated write-erase operations. A need also exists in such memories for the capability to erase in a practical manner single bits of data from the inscription data track.